The present invention relates to a power converting apparatus for converting dc power to AC power, or inverting AC power to dc power, and more particularly concerns controlling a voltage across a capacitor on a DC side of the power converting apparatus to control an AC voltage of three levels.
A usual three-level power converter operates in such a way that capacitors connected in series divides a DC voltage into two DC voltages to create three voltage levels, including high, intermediate, and low potentials, and switching devices of a main circuit are turned on or off to selectively supply the three voltage levels to an AC side of the power converter.
A method of controlling an AC output voltage in which a power converter is used as an inverter for converting DC to AC was proposed in "A Novel Approach to the Generation and Optimization of Three-Level PWM Waveforms," PESC '88 Record, April, 1988, pp. 1255-1262 (hereinafter referred to as document 1).
The document 1 proposes a dipolar modulation method in which positive and negative pulse voltages are alternately fed out via a zero voltage as a modulation method useful for waveform improvement and minute voltage control of the three-level inverter.
However, the three-level power converter exhibits a problem in that voltages of the capacitors for dividing the DC voltage into two levels (hereinafter referred to as the voltage dividing capacitors) are subject to becoming unbalanced.
The imbalance is due to unequal voltage sharing of the voltage dividing capacitors. The unequal voltage sharing is caused by a DC current component flowing into a serial connection (hereinafter referred to as the neutral point) of the voltage dividing capacitors. Such an undesirable flowing of the current component results from a difference in the capacitances of the voltage dividing capacitors, a dispersion of the AC voltages of the power converter (imbalance of the positive and negative pulses), or a distortion of the output current (imbalance of positive and negative current waveforms likely caused by superimposition of higher harmonics).
A technique for limiting the voltage imbalance of the voltage dividing capacitors was disclosed in Japanese Patent Laid-Open No. 2-101969 (1995), "Balance Control of DC Input Capacitors of NPC Inverter," Semiconductor Power Study Group, Association of Electric Engineers, SPC-91-37, June, 1991, pp. 111-120 (hereinafter referred to as document 2).